netdev - Re: [PATCH net-next 2/2] net: sched: protect against stack overflow in TC act

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Date:   Tue, 25 Jun 2019 10:06:00 +0100
From:   John Hurley <john.hurley@...ronome.com>
To:     Eyal Birger <eyal.birger@...il.com>
Cc:     Linux Netdev List <netdev@...r.kernel.org>,
        David Miller <davem@...emloft.net>,
        Florian Westphal <fw@...len.de>,
        Jamal Hadi Salim <jhs@...atatu.com>,
        Simon Horman <simon.horman@...ronome.com>,
        Jakub Kicinski <jakub.kicinski@...ronome.com>,
        oss-drivers@...ronome.com, shmulik@...anetworks.com
Subject: Re: [PATCH net-next 2/2] net: sched: protect against stack overflow
 in TC act_mirred

On Tue, Jun 25, 2019 at 9:30 AM Eyal Birger <eyal.birger@...il.com> wrote:
>
> Hi John,
>
> On Mon, 24 Jun 2019 23:13:36 +0100
> John Hurley <john.hurley@...ronome.com> wrote:
>
> > TC hooks allow the application of filters and actions to packets at
> > both ingress and egress of the network stack. It is possible, with
> > poor configuration, that this can produce loops whereby an ingress
> > hook calls a mirred egress action that has an egress hook that
> > redirects back to the first ingress etc. The TC core classifier
> > protects against loops when doing reclassifies but there is no
> > protection against a packet looping between multiple hooks and
> > recursively calling act_mirred. This can lead to stack overflow
> > panics.
> >
> > Add a per CPU counter to act_mirred that is incremented for each
> > recursive call of the action function when processing a packet. If a
> > limit is passed then the packet is dropped and CPU counter reset.
> >
> > Note that this patch does not protect against loops in TC datapaths.
> > Its aim is to prevent stack overflow kernel panics that can be a
> > consequence of such loops.
> >
> > Signed-off-by: John Hurley <john.hurley@...ronome.com>
> > Reviewed-by: Simon Horman <simon.horman@...ronome.com>
> > ---
> >  net/sched/act_mirred.c | 14 ++++++++++++++
> >  1 file changed, 14 insertions(+)
> >
> > diff --git a/net/sched/act_mirred.c b/net/sched/act_mirred.c
> > index 8c1d736..c3fce36 100644
> > --- a/net/sched/act_mirred.c
> > +++ b/net/sched/act_mirred.c
> > @@ -27,6 +27,9 @@
> >  static LIST_HEAD(mirred_list);
> >  static DEFINE_SPINLOCK(mirred_list_lock);
> >
> > +#define MIRRED_RECURSION_LIMIT    4
>
> Could you increase the limit to maybe 6 or 8? I am aware of cases where
> mirred ingress is used for cascading several layers of logical network
> interfaces and 4 seems a little limiting.
>
> Thanks,
> Eyal.

Hi Eyal,
The value of 4 is basically a revert to what it was on older kernels
when TC had a TTL value in the skb:
https://elixir.bootlin.com/linux/v3.19.8/source/include/uapi/linux/pkt_cls.h#L97

I also found with my testing that a value greater than 4 was sailing
close to the edge.
With a larger value (on my system anyway), I could still trigger a
stack overflow here.
I'm not sure on the history of why a value of 4 was selected here but
it seems to fall into line with my findings.
Is there a hard requirement for >4 recursive calls here?

John